Poison

331. Verify Preorder Serialization of a Binary Tree

发表于
Stack
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class Solution {
    public boolean isValidSerialization(String preorder) {
        String[] array = preorder.split(",");
        Deque<String> stack = new ArrayDeque<>();

        for (String str : array) {
            stack.push(str);

            while (stack.size() >= 3) {
                String top1 = stack.pop();
                String top2 = stack.pop();
                String top3 = stack.pop();
                if (top1.equals("#") && top2.equals("#") && !top3.equals("#")) {
                    stack.push("#");
                } else {
                    stack.push(top3);
                    stack.push(top2);
                    stack.push(top1);
                    break; // 注意需要 break 出 while 循环
                }
            }
        }

        return stack.size() == 1 && stack.peek().equals("#");
    }
}
Degree
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class Solution {
    public boolean isValidSerialization(String preorder) {
        // 所有节点的入度和与出度和应该相等
        String[] array = preorder.split(",");

        int degreeDiff = 1; // 给 root 预留的入度
        for (String s : array) {
            degreeDiff--; // 减去一个入度
            if (degreeDiff < 0) { // 在遍历节点过程中，出度减入度应该大于等于 0
                return false;
            }
            if (!s.equals("#")) {
                degreeDiff += 2; // 非空节点具有两个出度
            }
        }

        return degreeDiff == 0;
    }
}
Reference

331. Verify Preorder Serialization of a Binary Tree